Polyester polyols were
the first polyols used in the beginning of PU development, and are produced
by polycondensation of a diacid with excess diol (Figure 1.28).

Figure
1.28 - Obtaining polyol polyester

Difunctional monomers
are used to obtain a linear polymer; and monomers with functionality larger
than two as trimethylol propane and glycerin create ramified chains. The most
used acids are adipic and phthalics. Adipic acid based polyester polyols are
used in applications where flexibility is wanted, as in flexible foams and elastomers.
Phthalic acids (or phthalic anhydride) based polyols, have rigid chains and
are used in rigid foams and in high performance coatings (Table 1.8).

Table 1.8 - Typical properties of polyesters polyols

Application

Flexible
foam

Semi-rigid foam

Rigid foam

Shoe soles

Elastomers

Coatings

soft

hard

Structure

adipic acid,diethylene
glycol, trimethylol propane

adipic acid phthalic acid, 1,2-propylene glycol, glycerine

adipic acid, phthalic acid, oleic acid, trimethylol propane

adipic acid, ethylene
glycol, diethylene
glycol

adipic acid, ethylene
glycol, 1,4-butane diol

adipic acid,diethylene
glycol

phthalic acid,maleic acid, trimethylol propane

Average MW

2400

1000

930

2000

2000

2750

2450

OH number (mgKOH/g)

57 - 63

205 - 221

350 - 390

58-62

52 - 58

38 - 45

250 - 270

OH content (meq/g)

1.1

3.8

6.6

1.1

1.0

0.7

4.6

Average functionality*

2.7

3.8

6.2

2.1

2.0

2.0

11.3

Viscosity at 75°C (mPa.s)

950 - 1100

570 - 750

1300 - 1550

500 – 700

500 - 600

700 - 800

17000 a 150oC

Pour point (°C)

-12

-12

7

17 to 56

49 to 52

-9

90 to 100

Acid number

1.2

2.8

1.0

0.4

1.0

1.0

4.0

Density,
75°C
(g/cm)

1.15

1.15

1.1

1.15

1.17

1.12

1.24

*Average functionality
= PM x OH content (meq/g) / 1000

In polyester polyols
production process, the diol, triol, etc is first heated to a temperature of
60-90°C.
Then the dicarboxilic acid is added and removal of the reaction water begins.
For obtaining the targeted molecular weight the excess diol is calculated by
means of Flory Equation. Diol can be lost during removal of the water form the
condensation reaction and through side reactions (formation of ethers and aldehydes).
The amount of diol lost is dependent upon the processing conditions and upon
type of diol. The amount of diol lost must be empirically determined. Usually
the reaction is completed at temperatures up to 200°C.
Nitrogen, carbon dioxide, or vacuum is used to remove the water and to reach
the wanted conversion of 99.9%, and the resulting polyester should have an acid
number less than two. This conversion is necessary to minimize the presence
of residual carboxylic end groups that can reduce the reactivity. The polyesters
are composed of all possible oligomers raging from the monomers to high molecular
weight species: the molecular weight distribution follows a Frory probability.
The properties of the PU based polyester elastomers are governed mainly by the
overall molecular weight of the polyester and only to a minor degree by the
molecular weight distribution.

Acids, bases and compounds
of the transition metals can catalyze the esterification reaction. The dicarboxylic
acids also exert a limited catalytic effect. In practice catalysts are used
reluctantly because they cannot be removed and can have an undesirable effect
on the following PU reaction, since inorganic substances even in the smallest
quantities favor or retard de PU processing reaction. The p-toluenesulphonic
acid can be used as an accelerator and left in the polyester. In cases where
small amounts of catalysts do not later cause problems, compounds of tin, antimony,
titanium, lead and other metals, have proved especially effective. The amounts
added lie in the ppm range. Solid impurities are removed by hot filtration of
the finished polyester.

Usually aliphatic polyester
polyols used in flexible polyurethanes are based on polyadipates diols such
as ethylene glycol, diethylene glycol, propylene glycol, 1,4-butane diol, 1,6-hexane
diol, etc. The growth of the diol chain results in greater PU flexibility and
hydrolytic stability and reduction of polarity and glass transition temperature.
Polyester polyols used in PU elastomers (Chapter 6), based on acid adipic and
a glycol like ethylene glycol, 1,4-butane diol, 1,6-hexane diol, neopentyl glycol
or mixtures of them (Table 1.9), are crystalline products with melting point
between 50 and 60oC. The crystallinity can be reduced using mixed diols (as
1,4-butane diol and ethylene glycol) or mixed polyesters.

Lightly branched poly(diethyleneglycol
adipates), which are used mainly to make flexible foams, and a wide range of
adipates made with more than one aliphatic diol. These are used to make solid
and microcellular elastomers, flexible coatings and adhesives. Relatively low
cost polyester polyols, based on recovery materials are also available. Mixed
adipic, glutaric and succinic acid polyesters are made using purified nylon
waste acids (AGS acids). AGS acids are also hydrogenated to make a mixture of
1,4-butanediol, 1,5-pentanediol and 1,6-hexane diol, which is used to make polyadipates
having a low melting point. Mixed polyadipates from hydrogenated AGS acids are
used to make microcellular elastomers with good hydrolytic stability.

Table 1.9 - Polyester polyols
of MW = 2000

Structure

Solidification point (°C)

Viscosity at 75°C (mPa.s)

adipic acid + ethylene
glycol

52

540

adipic acid + ethylene
glycol + 1,4-butane diol

17

625

adipic acid + 1,4-butane diol

56

670

adipic acid + hexamethylene glycol + neopentyl glycol

27

640

In comparison with
PU based polyether polyols, the PU based polyesters are more resistant to oil,
grease, solvents and oxidation. They possess better properties related to: tension
and tear strength, flex fatigue, abrasion, adhesion and dimensional stability.
On the other hand PU based esters are more sensitive to hydrolysis and microbiological
attack. The high mechanical properties of PU based polyester can be explained
by the greater compatibility between polar polyester flexible segments and polar
rigid segments, causing a slower phase separation resulting in better distributed
small crystalline rigid blocks (Chapter 1.7).

The
hydrolysis stability of the ester linkage is inferior to that of the ether linkage
in the polyethers, and residual esterification catalysts accelerate the hydrolysis.
The hydrolysis resistance of the polyol polyester based PU increases with long
chain glycols (1,6-hexane diol) or long chain diacids (dodecanoic acid), as
a result of the largest hydrophobic portion and small amounts of ester groups.
The hydrolysis stability can be improved with additives that react with carboxylic
and alcoholic groups, formed during hydrolysis. These additives may be: oxazolines,
epoxy compounds, aromatic polycarbodiimides and aliphatic monocarbodiimides.
TPU's based polyester polyols are stabilized by addition of 1 to 2% in weight
of aromatic hindered carbodiimides, that react with the acid generated by ester
hydrolysis, which would act as catalyst of hydrolysis reactions (Figure 1.29).

Figure 1.29 - Reaction of carbodiimides
with carboxyl

Polymeric
polyester polyols are dispersions of vinyl polymers in polyadipate based polyol
polyester stabilized by a dispersant. Polymeric polyester polyols, containing
10 to 20% of vinyl polymers are used in shoe soles and flexible PU foams with
greater hydrolysis stability, higher hardness for same densities, more uniform
cellular structures, and better dimensional stability.

Another process for
production of aliphatics polyester polyols includes the ring opening polymerization
of e-caprolactone with glycols (Chapter 1). Polycaprolactone diols are produced
with diethylene glycol, 1,4 butane diol, neopentyl glycol or 1,6-hexane diol.
The polycaprolactone triols use trimetilol propane or glycerin /ethylene glicol,
and the tetrols are done with pentaerythritol. The polycaprolactone glycols
are produced with MW from 400 to 4000, hydroxyl number from 560 to 28 mg KOH/g,
and they have greater hydrolysis resistance and lower viscosity than the polyadipate
glycols of same MW. They are used in production of high resistance PU, modification
of resins, coatings, adhesives, shoe soles and orthopedic goods. Polycaprolactone
and polyadipate copolymers diols are usually liquids of low viscosity at room
temperature.

Aromatic polyester
polyols based on terephthalic or isophthalic acids are used in high performance
hard coatings and adhesives, and in polyurethane (PUR) or polysocyanurate (PIR)
rigid foams resistant to fire. In combustibility tests, the PIR and PUR foams
based on aromatic polyester polyols form a charred backbone, and in many formulations
reduces or eliminates the use of fire retardantes.

The polyterephthalate
glycols are usually obtained by polymerization of dimethyl terephthalate (DMT)
with ethylene glycol. Polyols with average equivalent weight of 181, functionality
2.3, hydroxyl number between 295 and 335 mg KOH/g, viscosities from 8,000 to
22,000 cP at 25°C,
can be used in rigid foams and foundry systems. The ones with average equivalent
weight of 238, functionality 2.0, hydroxyl number between 230 and 242 mg KOH/g,
viscosity from 2,700 to 5,500 cP at 25°C,
are used in PIR foams with minimum shrinkage and high weight retention. The
ones with average equivalent weight 167, functionality 2.0, hydroxyl number
between 315 and 350 mg KOH/g, viscosity of 1,300 to 3,000 cP at 25°C,
are used in appliance thermal insulation and other low viscosity applications.
Another polyterephthalates glycols obtaining process uses high molecular weight
poly(ethylene terephthalate) (PET) scraps of polyester fibers or soft drinks
bottles. The low molecular weight polyols are obtained by transesterification
of milled PET residues with propylene glycol or mixture of ethylene/propylene
glycols at 216°C
for about 6 hours.

The polyisophthalates
glycols are obtained by anhydride phthalic polymerization with glycols as diethylene
glycol. The poly(diethylene isophthalate) glycol with average equivalent weight
of 178 and 234, OH number of 230 to 330 mg KOH/g, viscosities from 2,000 to
4,500 cP at 25°C
are used in PUR and PIR foams. The ones with equivalent weight of 288, OH number
of 195 mg KOH/g, viscosity of 25,000 cP at 25°C
can be used in resins and prepolymers for coatings, adhesives, sealants and
elastomers, and also as additive in polyol polyether flexible foams to improve
fire resistance and adhesion characteristics. The poly(neopentyl isophthalate)
glycols with average equivalent weight of 510, OH number of 110 mg KOH/g are
used in adhesives, coatings and elastomers with excellent hydrolysis resistance.

The poly(oxytetramethylene)
glycol or polytetramethylene ether glycol (PTMEG) are manufactured by the cationic
polymerization of tetrahydrofuran (THF) (Figure 1.30). PTMEG's are linear chain
polyols with reactive primary hydroxyls and functionality of 2.0. PTMEG's of
molecular weights of 650, 1000 and 2000 (Table 1.10), are used in high performance
PU and TPU's elastomers, coatings and elastomeric fibers.

Figure 1.30 - Obtaining of PTMEG's

PTMEG's are solid, white, waxy at room
temperature, soluble in alcohols, esters, ketones and aromatic and chlorinated
hydrocarbons, and insoluble in aliphatic hydrocarbons and water. They have variable
solubilities for glycols: poly(oxypropylene) glycols and 1,6 hexanediol are
completely miscible whereas only 20% of 1,4 butanediol can be dissolved in PTMEG
1000 and less than 10% in PTMEG 2000. PTMEG polyols are hygroscopic and can
absorb 2% moisture in an unprotected environment. Gross amounts of water are
removed by azeotropic distillation with toluene, and further reduction can be
achieved by heating for several hours at 120-150oC under reduced pressure (less
than 20mm Hg). PTMEG's are stabilized with antioxidants to prevent degradation
during storage and normal handling. However, prolonged heating in the presence
of air at 50-60°C will result in partial oxidation and
degradation, and thermal decomposition will occur, in absence of air conditions
at 210-220°C.

Castor oil (ricinus
oil) is a pale yellow and viscous liquid (gardner viscosity U-V to 25°C), derived
from the bean of the castor plant (ricinus communis) that occurs in all tropical
and subtropical regions. It is triglyceride of fatty acids that contains 87-90%
of ricinoleic acid (cis-12-hydroxyoctadec-9-enoic acid), with a hydroxyl number
of 163 mg KOH/g, and average functionality of about 2.7 (Figure 1.31). Castor
oil and its derivatives are used as polyols for the PU preparation mainly and
in coatings, adhesives, and casting compounds with excellent hydrolytic stability,
shock absorbing and electrical insulation properties. They also have been found
to be very useful in the preparation of flexible, semi-rigid and rigid PU foams,
resistant to moisture, sock absorbing, and with low temperature flexibility.
The products with high purity are the recommended for PU's applications.

Figure 1.31
- Castor oil polyol

Transesterification
of castor oil with polyhydroxylated compounds like glycerin trimethylolpropane,
or propylene glycol results in polyols with higher or lower functionality. Transesterification
with glycerin forms a trifunctional polyol mixture of mono and diglycerides
(Figure 1.32), with OH number of 300 mg KOH/g. Castor oil based polyols with
OH number of 310 mg KOH/g are used to promote pentanes blowing agent solubility,
in rigid foams systems, with good thermal dimensional stability.

Several
polyols with hydrocarbon structure are found in the marketplace. The main advantage
of the PU based hydrocarbon polyols is the high resistance to hydrolysis, acids
and bases. PU's based saturated hydrocarbon polyols have high temperature stability
and are used in automotive electronic encapsulation. An important HTPB is obtained
by free radical polymerization of butadiene, initiated by hydrogen peroxide
and an alcohol as diluents (Figure 1.32).

Figure 1.32 - HTPB
obtaining reaction

Due
to the free radicals process it has ramifications in polymeric chain, and functionality
slightly higher than two (2.1<2.3). HTPB possesses reactive allylic primary
hydroxyl end groups, molecular weight of 2,800 and hydroxyl number of 46 mg
KOH/g. HTPB's hydrophobic polymeric chains form Pus with exceptional hydrolysis
stability, and its low humidity degree (<300 ppm), minimize or eliminate the
previous drying. Due to its very low glass transition temperature the PUs formed
have excellent elastomeric properties at extremely low temperatures. They possess
great capacity to accept fillers as asphalt, aromatic and paraffinic oils, pentanes,
plasticizers, carbon black, etc. HTPB microstructure is 60% of 1,4-trans, 20%
of 1,4-cis and 20% of 1,2-vinyl insaturations (Figure 1.33) that turn possible
further vulcanization and chemical modifications.

Figure 1.33 - HTPB
microstructure

Another
type of HTPB is obtained by anionic polymerization of butadiene initiated by
sodium naphthalene, and terminated by reaction with ethylene or propylene oxides,
following by hydrolysis, resulting in the formation of groups OH primary or
secondary, respectively. This commercial HTPB's have functionality 2.0, molecular
weights between 2000 and 5000, and usually possess secondary hydroxyl groups.
They present microstructure with high quantity of 1,2-vinyl double bonds, which
turns them extremely viscous (waxy) at room temperature. Due to the 2.0 functionality,
these polybutadiene diols can be used in the thermoplastic elastomers (TPUs)
(Chapter 6.3), with excellent hydrolysis and chemical stability, and insulating
properties.